Automatic gain control



Patented Aug. 27, 1946 UNITED STATES PATENT OFFICE AUTOMATIC GAIN CONTROL Delaware Application January 28, 1942, Serial No. 428,538

My invention relates to automatic gain control circuits and particularly to gain control circuits containing no long time constant filter. The invention is especially adapted for use in systems such as the rotating radio beam system described in Luck Patent 2,208,376, issued July 16, 1940, wherein it is desired to remove certain low frequency variations of the order of from 1 to 6 cycles per second, for example, and to remove, also, comparatively high frequency variations which may occur at various random frequencies lying between 40 cycles per second and 100 cycles per second for instance.

In the system described in the above-mentioned Luck patent, an antenna figure-of-eight radiation pattern is rotated at the rate of 60 rotations per second, in one particular example, and the pattern is provided with a reference mark whereby an airplane may determine its position b means of a suitable receiver carried thereby. In this particular system the metal propellers of the airplane introduced a modulation at the receiver having a frequency differing only a few cycles from the 60 cycle frequency introduced by the rotating antenna pattern. Thus there results a modulation component equal to this difference frequency and the cathode ray pattern on the receiver tube tends to bounce at this low frequency rate.

It is an object of the present invention to pro- Vide an improved automatic gain control circuit for avoiding the above-described undesirable effects in a rotating radio beacon.

A further object of the invention is to provide an improved automatic gain control circuit which removes both low frequency and comparatively high frequency undesired signal variations.

A still further object of the invention is to provide an improved means for and method of developing an automatic gain control voltage,

In one of the preferred embodiments of the invention the signal having the undesired amplitude variation is detected and applied to a clipping circuit which clips at a level that varies with the value of the instantaneous amplitude as determined by the undesired modulation. This may be accomplished by means of a triode having a grounded grid which, together with a resistor-ca.- pacitor circuit, is so connected to the detector that the half-wave pulses in its output periodically cause a flow of grid current during which period the condenser receives a charge, and at the end of which the condenser partially discharges. During the flow of grid current the half-wave pulses are clipped off at a certain level. The flat- 9 Claims. (Cl. 250-'--20) topped pulses thus obtained may be passed through a filter having a fast time constant to obtain the desired gain control voltage since the filter need hold a charge only for the comparatively short time between successive flat-topped pulses rather than for the time between the peaks of rectified sine waves as would be the case in the usual gain control circuit.

The invention will be better understood from the following description taken in connectionwith the accompanying drawing in which Figure 1 is a circuit diagram of oneembodiment of the invention, 7

7 Figures 2a to 2d are curves which are referred to in explaining the invention,

Figure 3 is a circuit diagram of another embodiment of the invention,

Figure 4 is a series of curves which are referred to in explaining the invention.

Like parts in the several figures are indicated by similar reference characters.

In Fig. 1, one embodiment of the invention is shown applied to the receiver of a rotating beacon system. The receiver, the front end of which is indicated at 9, includes a vacuum tube 10, which may be of the pentode type for amplifying the received signal and supplying it through a conductor H to a cathode ray tube indicator circuit (not shown) and through a transformer I2 to my improved automatic gain control circuit. The latter circuit includes a detector comprising diodes l3 and it and a load or output resistor l6 across which appears the voltage c (Fig, 2a).

A vacuum tube ll having a plate IS, a control grid l9 and a cathode 2! is connected across the output resistor It, theplate l8 preferably being connected through a resistor 22 to the cathode or positive end of resistor It, and the cathode 2| being connected through a conductor 23 to the plate or negative end of resistor It. The grid 19 is grounded.

A condenser 24 is connected between the plate l8 and grid l9 and a resistor 26 is connected b tween the grid !9 and the cathode 2|. The capacity of the condenser preferably is small enough to make its impedance at cycles per second large compared with that of the grid-cathode impedance of tube IT and the resistor 26 in parallel.

The automatic gain control voltage may be obtained at the cathode 2| and applied to a grid of the amplifier tube H1 through a conductor 21 and through a comparatively short time constant filter comprising a series resistor 28 and a shunt condenser 29. In this case the voltage 21 appear- 3 ing across resistor 26 is utilized (after filterin by filter2829) as the gain control voltage.

The operation will be described with reference to Figs. 2a and 2b which show the voltages e and ei, respectively. -As a half sine wave pulse of the voltage e increases in amplitude from its zero value, the current flow through the resistor 26 increases correspondingly up to the point where the grid l9 goes positive with respect to the cathode 2! to cause a fiowofgrid current; It may aid in understanding the operation to note that Thus the amplitudes of the fiat pulses e1 in Fig. 2b are proportional to the amplitudes of the pulses e in Fig. 2a whereby any undesired amplitude variations (either fast or slow) of pulses e may be substantially eliminated.

For some applications of the invention the resistor 22 may be omitted whereby the voltage m the series connected elements 2 24 and .26' may be looked upon as a voltage divider connected across the load resistor l6. (i

As soon as the grid l9 goes positive, this point being indicated by the broken line 3| in Fig. 2q. the current fiow through resistor 26 becomes sub stantially constant to produce the fiat-topped portion of a pulse of voltage e1.

This current flow remains constant until the half-wave pulse of voltage e decreases to a value where the grid I9 is no longer positive with respect to the oathode 2 l and the flow of grid current ceases. The condenser meanwhile has been charged by the flow of grid, current, and as the applied voltage e decreases, condenser 24 begins to discharge through the circuit 22, i6, 26.

Since the current flow through resistor 26is reversed during this short discharge time, the cathode end of resistor 26 goes positive momentarily with respect to ground. The gain control voltage applied to the grid of tube do, however, i s alwaysnegative with respect toground because of the smoothing action of the filter zap- 29. This is indicated by the dottedline Dortion x of Fig. 2b.

When voltage e drops to or close to zero, the

cathode 2| will be at the same or nearly the same potential as the upper end of resistor 22 and, therefore, will be positive with respect to the grid l9 asa result of thecharge on condenser 24 whereby the tube I1 is biased to cut-off. Now, when thevoltage starts to rise again On the next pulse, the tube I is still cut-off so that there is novoltagedrop in resistor 22 due to plate currentfiow. Consequently, the potential on condenser 24 never falls .to a value as low as it would otherwise, since at plate current cut-off of tube I'Lthe potential across condenser 24 and resistor 2 6 in series is'increased by the value of the potential acrossresistor 22 at a time just before plate current cut-off occurs.

As the voltage e increases further in amplitude du g't is second half -wave pulse, the previous y described action is repeated, but the clipping level'(or point. at which grid current starts to flow) is proportional to the amplitude of the preceding half-wave pulse of voltage e, Specifically, the charge on condenser .24 is proportionalto the charging voltage 6, and the resulting voltage across condenser 24 opposes the applied Voltage '6. As soon as the rising voltage 6 becomes greater than the condenser voltage, then, and onlythen, does current start to flow through the circuit 2426 so that the grid [9 goes positive with respect tocathode 2|. It is evident that the smaller the amplitude of a pulse of voltage 6, the smaller the Voltage on condenser 24 during .the next following pulse, and the less rise there. is in the voltage of the said next pulsewhen V the grid current starts to flow, and also with smaller values of e, the anode-cathode potential of tube ll is reduced with a corresponding lowering of grid-cathode resistance so that clippingstarts at a lower. value.

will have a wave shape such as indicated'in Fig. 201. It will be apparent that the use of resistor .22 will result in a gain control output that will require a minimum amount of filtering.

I .iThe plate-cathode voltage 62 may be utilized for gain control in place of voltage e, if desired. In that case the ground connection is removed from thegrid l9 and the plate i8 is grounded instead. If the time constant of condenser, 25

and resistor '25is fast, the volta e e2 will have the wave shape shown in Fig. 20; if the time constant is long the wave shape of voltage at will more closely resemble that of voltage 61 shown in Fig. 2b.

In the embodiment of Fig. 3, a multivibrator A!) is utilized rather than the grounded grid triode of Fig. l. The detector output e is applied constant.

to the plates of multivibrator tubes 4! and 42 through their plate resistors 53 and M. I multivibrator preferably is symmetrical so that it produces rectangular pulses of the same width at the plates of tubes 4! and 42 as shown in Fig.

4; at a and b, respectively. These pulses will have an amplitude that is proportional to the applied plate voltage e. They are combined by passing them through diodes t6 and Hand through a common resistor 48 whereby the signal shown .at

' c in Fig. 4 is supplied over the conductor 21 to the filter 2829. It will be apparent that, as in the case of the circuit shown in Fig. 1, very low frequencies can be substantially eliminated even though the filter 28-29 has a fairly fast time In some uses the filter 2829, in either Fig. 1 or Fig. 3, may be omitted, particularly since there is some filtering due to tube capacity as indicated at Cg in Fig. l, for example. -From the foregoing, it will be apparent that I have provided an improved gain control system that is effective to reduce or eliminate very slow signal variations without employing any filter having a slow time constant. Thus the system is capable of removing rapid signal variations as well as the slow ones.

I claim as my invention: 7

1. In combination, an amplifier for amplifying asignal having an undesired modulation thereon,

arectifier coupled to said amplifier to produce a rectified output signal comprising voltages pulses that have amplitudes determined by said modulation, means for clipping said pulses at a voltage level which varies in accordance with variations in the amplitudes of said pulses, and means for applying said clipped pulses in negative feedback relation to said amplifier for controlling its gain. 7 2. The invention according to claim 1 wherein said clipped pulses are applied to saidlamplifier through a filter having a time constant/that is fast with respect to the period of the undesired modulation.

' 3.' In'co'mbination, an amplifier for amplifying a signa1 having an undesired modulation thereon, I a, rectifier havinga load resistor coupled to said" amplifier to supply thereto a rectified output signal comprising voltage pulses that have amplitudes determined by said modulation, a vacuum tube having a cathode, a plate'and 'a grounded grid/said plate and cathode being connected to the positive and negative ends of said loa'd'resistor, respectively, a condenser connected between said plate and grid, a resistor connected between said grid and cathode, and means for applying the potential appearing on said cathode to said amplifier for automatic gain control.

4. The invention according to claim 3 wherein the connection from said plate to the positive end of said load resistor includes a resistor.

5. In combination, an amplifier for amplifying a signal having an undesired modulation thereon, a rectifier having a load resistor coupled to said amplifier to supply thereto a rectified output signal comprising voltage pulses that have amplitudes determined by said modulation, a multivibrator comprising two vacuum tubes each having a plate connected through its plate resistor to the positive end of said load resistor and each having a cathode connected to the other end of said load resistor, means for combining the rectangular voltage pulses appearing at said plates, and means for applying said combined pulses to said amplifier for automatic gain control.

6. In combination, an amplifier for amplifying a signal having an undesired modulation thereon, a rectifier having a load resistor coupled to said amplifier to supply theret a rectified v output signal comprising voltage pulses that have amplitudes determined by said modulation, a multivibrator comprising two vacuum tubes each having a plate connected through its plate resubstantially the same width.

8. In combination, a amplifier for amplifying a signal having an undesired modulation thereon, a rectifier coupled to said amplifier to produce a rectified output signal comprising voltage pulses that have amplitude determined by said modulation, means for converting said pulses into flattopped pulses having peak values at a voltage level which varies in accordance with variations in the amplitudes of said pulses, and means for applying said fiat-topped pulses in negative feedback relation to said amplifier for controlling its gain.

9. The invention according to claim 8 wherein said fiat-topped pulses are applied to said amplifier through a filter havin a time constant that is fast with respect to the period of the undesired modulation.

LOWELL E. NORTON. 

